The present application relates to RAN overload control. Important background information to the invention may be found in the 3rd Generation Partnership Project (3GPP) specification TS 22.011, section 4.3.4.
The RRC protocol layer provides functions such as broadcasting system information in a RAN. Based on the characteristics and uses of this information, the information elements are grouped together into a master information block (MIB) and different system information blocks (SIBs). The MIB is transferred on a broadcast channel every 40 ms and is repeated within 40 ms. A user equipment (UE) acquires the MIB to decode a shared channel. The MIB contains a downlink system bandwidth, a physical HARQ indicator channel, and a system frame number, wherein HARQ is an acronym for hybrid adaptive repeat and request.
SIB1 is the first system information block to be transferred after transferring MIB. It is scheduled in a fixed manner with a periodicity of 80 ms and is repeated within 80 ms. SIB1 contains cell-access-related information, information for cell selection, a frequency band indicator, etc. In addition, SIB1 contains SI-window length and a system information value tag. SIB1 is transmitted in a system information block message and the other SIBs are transmitted in system information (SI) messages. Each SI message is transmitted periodically in time domain windows, i.e. in SI-windows, and SI windows for different SI messages do not overlap. The length of SI-window defined in SIB1 is the same for all SI messages.
SIB2 contains radio resource configuration information common to all UEs. In more detail. SIB2 includes access barring information, radio resource configuration of common channels, and a lot of other information. SIB10 contains earthquake and tsunami warning system (ETWS) primary notification and SIB11 contains ETWS secondary notification by which users of UEs can be warned in case of an earthquake or a tsunami.
When the system information is updated, SI messages are repeated during modification periods. During the first modification period, the system information modification indicator is sent in a paging message to an UE, and during the next modification period, the RAN transmits the updated system information to the UE.
The UE verifies that its system information remains valid by either checking systemInfoValueTag in SystemInformationBlockType1 after the modification period boundary. Alternatively, the UE attempts to find systemInfoModification indication during a modification period. If no paging message is received by the UE during the modification period, the UE assumes that no change of system information will occur during the next modification period. If the UE is in RRC_CONNECTED mode and it receives the paging message, it deduces from the presence or absence of systemInfoModification whether a change of system information will occur in the next modification period or not.
As mentioned above, the SIB 2 includes the access barring information. In more detail, the SIB2 is used in access class barring (ACB) which is today's method to implement access barring in LTE networks. ACB comprises a number of parameters to indicate which of the access classes are currently barred and each UE should compare its access class to the barred access classes.
The maximum modification period during which the access barring information reaches UEs is calculated as: modificationPeriodCoeff×defaultPagingCycle. For example, the maximum modification period could be in radio frames (rf): 16×256 rf=4096 rf which requires 40960 ms. Thus, the access barring information reaches the UEs quite slowly. The delay associated with reading the updated system information (the access barring information) is not acceptable in some overload scenarios, in terms of being able to protect the RAN from excessive access attempts. The sudden surge in access attempts may happen because of an earthquake or a tsunami, but more probably because of smart metering, fleet management, civilian surveillance, etc. applications.